1. Field of the Invention
The present invention relates to separators, used as constituent parts of solid polymer fuel cells that are used for automobiles using electric power as a direct driving source and a small-scale generation systems, and a method for producing the same, and solid polymer fuel cells formed with the separators.
2. Description of the Related Art
Use of electricity-driven automobiles, using solid polymer fuel cells that utilize hydrogen in place of currently used internal combustion engines that utilize fossil fuel, and replacement central generation systems with dispersed cogeneration systems has been encouraged, because the importance of an unpolluted environment has been recognized.
In order for the new technologies to be widely and commonly utilized, development of technologies related to cutting the cost of solid polymer fuel cells and making the fuel cells highly reliable, as well as providing fuel supply systems, must be advanced.
The success of development of solid polymer materials for fuel cells has rapidly advanced the development of fuel cells for electric automobiles in recent years.
Solid polymer fuel cells differ from conventional alkali fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells and the like. The solid polymer fuel cells are fuel cells that comprise organic films which are classified as a hydrogen-ion-selective-permeation type as an electrolyte. The solid polymer fuel cells are based on a system that generates power by using as a fuel pure hydrogen, a hydrogen gas obtained by modifying alcohols, or the like, and electrochemically controlling the reaction of hydrogen with oxygen in the air.
Since a solid polymer film firmly fixes an electrolyte in the film even when the film is thin, accurate control of a dew point within the cells makes the electrolyte function as an electrolyte. Accordingly, the solid polymer fuel cells do not require the use of a flowable medium such as an aqueous electrolyte and a molten salt electrolyte. The solid polymer fuel cells are therefore characterized by that the cells themselves can be compact and simple.
The solid polymer fuel cells have, as a unit cell, a sandwich structure comprising a separator having a hydrogen flow path, a fuel electrode, a solid polymer film, an air (oxygen) electrode and a separator having an air (oxygen) flow path. Practically, the solid fuel cells are formed from a stack obtained by stacking the unit cells. Accordingly, both sides of the separator each have an independent flow path. That is, one side has a hydrogen flow path, and the other side has a flow path for air and the water thus produced.
A carbon material which can fully display corrosion resistance and durability in an environment where the operation temperature is not very high and in which optional flow shapes can further be formed is used, after processing such as machining, as a constituent material of the solid polymer fuel cells that are operated in a temperature region of up to the boiling point of a cooling aqueous solution. Development of technologies for using stainless steel or titanium as the above constituent material has been advanced in order to reduce the cost and size and, namely, in order to make the separator thin.
There has heretofore been used a stainless steel, for fuel cells, that can be operated in a molten carbonate environment requiring a high corrosion resistance, as disclosed in Japanese Unexamined Patent Publications (Kokai) No. 4-247852, No. 4-358044, No. 7188870, No. 8-165546, No. 8-225892 and No. 8-311620, and the like references.
Moreover, Japanese Unexamined Patent Publications (Kokai) No. 6-264193, No. 6-293941 and No. 9-67672, and the like, disclose solid electrolyte fuel cell materials that can be operated at temperatures as high as several hundred degrees centigrade.
Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 10-228914 discloses a separator for a fuel cell that has been developed for the purpose of decreasing the contact resistance against the electrode of the unit cell, and that is characterized by press forming a stainless steel (SUS 304) to form a stretch formed portion composed of many recesses and protrusions in the inner peripheral portion, and forming a gold plating layer from 0.01 to 0.02 xcexcm thick on the end face of the stretched tip of the stretch formed portion. The patent publication further discloses technologies for the method of using the separators for fuel cells, which comprises allowing each of the separators to intervene between stacked two unit cells, and arranging the electrodes of the unit cell so that each of the electrodes is in contact with the gold plating layer formed on the end face on the stretched tip of each of the stretch formed portions, resulting in the formation of reaction gas paths between the separator for fuel cells and the electrode. Moreover, Japanese Unexamined Patent Publication (Kokai) No. 5-29009 discloses xe2x80x9cperforated bipolar platesxe2x80x9d in a corrugated shape obtained, by pressing, at low cost.
However, the present inventors have actually prepared solid polymer fuel cells on the basis of the disclosed technologies, and found that the disclosed technologies have the following four technological problems.
a) The alloy components of SUS 304, that is a general-purpose steel, sometimes become unsatisfactory as stainless steel separators in the environment of solid polymer fuel cells where long-term durability is required. In order to take countermeasures, the contents of Cr, Ni, Mo, etc. must be increased.
b) For a stainless steel in which the contents of Cr, Ni, Mo, etc. are increased, when the stainless steel is plated with gold by wet gold plating alone, a passivated oxide film of the stainless steel remains without being completely reduced during gold plating. As a result, an interlayer resistance is sometimes produced between the stainless steel and the gold plating layer to cause a power loss. In order to take countermeasures, a noble metal must be allowed to adhere to the stainless steel while the passivated oxide film of the stainless steel is being removed.
c) A form of a separator obtained by press forming a stretch formed portion composed of many recesses and protrusions in the inner periphery portion can be assumed as the form of a separator. However, when an attempt is made to work a separator part having flat portions around the periphery, ductile cracks are formed in the stretch formed portion composed of recesses and protrusions. Moreover, the stainless steel in which the contents of alloy components are increased, to improve long term reliability, is difficult to press form into a separator having the above form because the steel shows lowered workability compared with that of SUS 304. Furthermore, when the separator has a cross section in a corrugated shape, the area contacted with an electrolytic film of the separator is decreased to make the fuel cell characteristics poor.
d) When the form of a separator having a stretch formed portion composed of many recesses and protrusions in the inner peripheral portion is assumed as the form of the separator, a reaction gas freely flows along a space between the separator and the electrode in the resultant structure. There arise the following problems in this case: the gas does not flow uniformly from the gas inlet to the gas outlet, and the reaction efficiency lowers, the gas flow speed is low, and water formed on the oxygen side is discharged with difficulty.
The present inventors have already proposed means for solving the problems mentioned in a) and b) in Japanese Unexamined Patent Publications (Kokai) No. 2000-256808 (Application No. 11-62813) and No. 2000-006713 (Application No. 11-170142).
In view of the problems in c) and d) mentioned above, an object of the present invention is to provide press formable separators for solid polymer fuel cells and method for producing the same, and further provide highly durable solid polymer fuel cells, at low cost, in which the separators are used.
In order to solve the above problems, the present inventors have examined in detail the material behavior at the time of press forming separators on the basis of the functional principle of the solid polymer fuel cells, and achieved the present invention based on the results. The gists of the present invention are as described below.
(1) A separator for solid polymer fuel cells, comprising a flat peripheral portion and continuous channels in the central portion composed of protruded portions and recessed portions which provide flow paths for gases on the front surface and the back surface of the central portion, is characterized by the channel end portions of each of the channels being tilted.
(2) The separator for solid polymer fuel cells according to (1) mentioned above, wherein the tilting angle xcex8 (degrees) of the channel end portions is varied from channel to channel.
(3) The separator for solid polymer fuel cells according to (1) or (2) mentioned above, wherein the depth H (mm) of each channel is made equal to or less than a value calculated by the following formula:
H=2xc3x97Wxc3x97(EL/YS)1.01xc3x97(R/T)0.318xc3x97(1xe2x88x92W/P)2.66 
wherein P (mm) is a channel pitch of the channel, R (mm) is a radius of the shoulder portion of the channel, W (mm) is a length of the parallel portion of the channel, t (mm) is a thickness of the separator, EL (%) is an elongation of a material used for the separator and YS (kgf/mm2) is a yield stress of the material used therefor.
(4) The separator for solid polymer fuel cells according to any one of (1) to (3) mentioned above, wherein the tilting angle xcex8 (degrees) of each channel end portion is made equal to or less than a value calculated by the following formula:
xcex8=90xc3x97(EL/YS)0.372xc3x97(R/t)0.270xc3x97(W/t)xe2x88x920.265 
wherein R (mm) is a radius of the shoulder portion of the channel, W (mm) is a length of the parallel portion of the channel, t (mm) is a thickness of the separator, EL (%) is an elongation of a material used for the separator and YS (kgf/mm2) is a yield stress of the material used therefor.
(5) The separator for solid polymer fuel cells according to (1) mentioned above wherein, in the transverse cross section of a gas flow path formed by the repetition of a protruded portion and a recessed portion of the channel, the outside surface of each protruded portion and that of each recessed portion each have a flat portion and the shoulder portion of each protruded portion and that of each recessed portion each have a curved portion having a constant curvature.
(6) The separator for solid polymer fuel cells according to (5) mentioned above, wherein the connecting portion of the flat portion and the shoulder portion has a bent portion.
(7) The separator for solid polymer fuel cells according to (5) or (6) mentioned above, wherein the entire upper bottom portion and the entire lower bottom portion on the inside surfaces of the protruded portion and the recessed portion are curved with a constant curvature.
(8) The separator for solid polymer fuel cells according to any one of (5) to (7) mentioned above, wherein the following formula is satisfied:
ELxe2x89xa750xc2x7t/R 
wherein R (mm) is a curvature of the shoulder portion, or the upper bottom portion and the lower bottom portion, EL (%) is an elongation of the above material, and t (mm) is a thickness of the above plate.
(9) The separator for solid polymer fuel cells according to any one of (1) to (8) mentioned above, wherein the separator has a seal member that seals both flat faces of the peripheral portion.
(10) The separator for solid polymer fuel cells according to any one of (1) to (9) mentioned above, wherein the cross-sectional area of the channel increases toward the downstream end of the gas flow path.
(11) The separator for solid polymer fuel cells according to any one of (1) to (10) mentioned above, wherein the separator is made of a stainless steel or titanium.
(12) A method for producing a separator for solid polymer fuel cells, comprising press forming with a mold having a configuration similar to the external shape of the separator for solid polymer fuel cells according to any one of (1) to (11) mentioned above.
(13) Solid polymer fuel cells comprising the separators for solid polymer fuel cells according to any one of (1) to (11) mentioned above.